1. Field of the Invention
The present invention relates to a color copying machine for exactly copying a color material on a subject paper, and more particularly to the color copying machine which provides a color correcting circuit.
2. Description of the Related Art
In recent days, a great remark has been placed on a color processing technique which may apply to exact color copying of a subject material or a display image. A color copying machine provided with such a color processing technique known by the inventors of the present application is basically arranged to a color correcting unit, an image reader, a printer, and a memory. In addition, some controllers for various controls may be included in the known copying machine. However, the description herein does not concern with such controllers, because they have no substantial concern with the present invention.
In operation, the image reader serves to read a material to be copied. The reader may be arranged on the same principle as the commercially available color scanner. The read data about the material is corrected by the color correcting unit and is printed as an exact color copy by the printer. The printer is arranged on the same principle as the commercially available color printer.
In actual, however, the known color copying machine does not enable to exactly copy any kind of color material. This is because though the principle of subtractive mixture of color stimuli is applied to the color development of color printing, the known copying machine does not often provide a capability of establishing an additive law and a multiplicative law which are the fundamental principles of the subtractive color mixture. To overcome this shortcoming, the color copying machine performs the color correction operation with the color correcting unit.
As a method commonly used for the color correction, a 3.times.3 matrix operation represented by the following expression (1) is performed with respect to an input signal (r, g, b) each item standing for red, green and blue. ##EQU1## wherein (R, G, B) represents a corrected signal and all to a.sub.33 denote coefficients for color correction. Those coefficients are set when designing or shipping the products and are constantly stored in the memory.
The color correcting unit is reading the correction coefficients as accessing the memory so that the data is output to the printer as sequentially performing the operations of the expression 1. The color-corrected output can be obtained from the printer.
Next, the principle of the color correction based on this method will be briefly described later. The subtractive mixture of color stimuli done in the color copying machine is arranged on the assumption that the used inks (cyan, magenta, yellow) have the characteristics of independently absorbing the red, the green and the blue.
However, the actually used inks have their characteristics mutually effecting on red, green and blue. To copy each correct color on the principle of the subtractive color mixture, it is necessary to define a printing density of one ink as relating it with two other color densities.
The expression 1 indicates such a mutual influencing relation. By performing the color correction by means of the expression 1 having the properly-defined coefficients substituted therefore, it is possible to eliminate the effect of one color ink on the other colors. This results in allowing the printed output to approximate to the printed colors used in the ideal inks.
The expression 1 indicates the method for color correction based on the 3.times.3 matrix, while the color correction may be made possible on a matrix containing high degree items as shown in the expression 2. ##EQU2##
The color correction based on the expression 2 may offer higher accuracy than that based on the expression 1, which has been described in Joji Tajima, "Color Masking (II)", Proceedings of Image Electronics, Vol. 18, No. 2, P. 44-48.
In the actual color correction, the expression 1 or 2 is implemented by hardware or software. For the implementation, the proper digital signal processor (DSP) or CPU having a high capability of numerical operations is often used. Or, another CPU functioning as controls may be used.
The coefficients indicated in the expressions 1 and 2 has to be set as considering change in a wide range such as initial design of the products, adjustment in shipping and maintenance for aging. For setting the coefficients in the former two cases, it is possible to take a considerably long time. But, at the maintenance stage, it is not possible to take a long time to terminate the work. In any of those cases, it goes without saying that the variation of the set values has to be lowered to a minimum. That is, the setting of those coefficients indicated in the expression 1 and 2 is required to be implemented, because they define the color reproduction provided by the color machine.
For setting those coefficients, the inventors of the present application know that the repetition of observational evaluations or the least squares method has been used.
If the repetition of observational evaluations is used for setting those coefficients, it may be advantageous in that the color reproduction error for human visual perception may be lowered to a maximum. To properly set the coefficients with this method, it needs to consume a quite long time. In addition, this setting method entails the variation resulting from the difference of human perceptions among inspectors, It means that this setting method cannot make sure of a constant color reproduction accuracy when changing each product model or updating each lot.
As such, this setting method has difficulty in obtaining satisfactory results at the initial design, the adjustment in shipping and the maintenance stage.
In the case of using the least squares for setting the coefficients, in a sense, the set coefficients are made non-variable and optimized.
Now, the method of setting the coefficients on the least squares will be described. Concretely, the multiple regression analysis, which is one kind of the least squares of multiple variables, is employed in this description.
The multiple regression analysis consists of numerical calculations which serve to define a coefficient a.sub.i for describing a depending variable Y according to the definition of the expression 3 by using a plurality of independent variables X.sub.1 to X.sub.n. EQU Y=a.sub.1 X.sub.1 +a.sub.2 X.sub.2 +. . . +a.sub.n X.sub.n +C (Expression 3 )
wherein a.sub.i denotes a coefficient against an i-th independent coefficient X.sub.i (i=1 to n), C denotes a constant, and n denotes a number of independent variables.
Assuming that the independent variables X.sub.1 to X.sub.n denote r, g and b indicated in the expression 1 and Y denotes R therein, by using the multiple regression analysis, the coefficients a.sub.11 to a.sub.13 may be obtained.
The procedure for obtaining the coefficients will be performed as follows:
(Step 1) Measure a color sample with a colorimeter and read it from an image input unit.
(Step 2) Print the color sample data and measure the printed result with the colorimeter.
(Step 3) Collect a lot of pieces of measured data and perform the multiple regression analysis by using the measured data of the original color sample and the measured data of the printed result.
(Step 4) Obtain those coefficients as changing Y to G and B.
It is well known that the multiple regression analysis has been established as a numerical calculation program used in a general computer. As such, if several pieces of data are allowed to be collected, with this method, the use of this method is advantageous in that the coefficients can be defined at high speed and the defined coefficients are made non-variable.
However, it has been traditionally necessary to measure a lot of color samples each color at one time. This results in disadvantageously consuming a quite long time for measuring those color samples.
As noted above, the color correction coefficients are set in some stages such as the standard setting at the initial design stage of the products and the adjustment for the products in shipping. The foregoing known methods provides no capability of setting such color correction coefficients as achieving a high color reproduction performance for a quite short time. Hence, heretofore, the setting of the color correction coefficients is not necessarily accurate.
Further, to correct for the aging of the product performance after shipping, it is preferable to reset the color correction coefficients at the maintenance stage of the product. The color correction coefficients cannot be newly rewritten in the light of the structure of the used memory. No disclosure has been proposed of the method for setting less variable and highly reliable coefficients for a short time. As such, in actual, the resetting of the coefficients has been made impossible.